1. Field of the Invention
The present invention relates generally to a throttle control system for an automotive internal combustion engine, which controls a throttle valve angular position depending upon operational magnitude of an accelerator, such as an accelerator pedal, accelerator handle and so forth, through a electric or a hydraulic servo mechanism. More specifically, the invention relates to a throttle control system with a fail-safe mechanism which is durably perform fail-safe operation at the occurrence of failure in throttle control.
2. Description of the Background Art
One of the prior proposed throttle control system has been disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa 59-190440. In this prior proposed system, an accelerator is connected to a detector which is designed to detect magnitude of operation of the accelerator to generate an electric command for driving a throttle valve, via a linkage, such as a linkage wire. The throttle valve is coupled with an electric servo mechanism including an electric motor which is responsive to the electric command for rotatingly driving throttle valve at an angular position corresponding to operation magnitude of the accelerator. The system is further provided with a throttle return mechanism for driving the throttle valve back to its initial position (fully closed position) in response to releasing of operational force on the accelerator. In the shown system, the throttle return mechanism is associated with the aforementioned linkage and, in turn, associated with a rotary shaft of the throttle for exerting force to return the throttle valve to the initial position. This throttle return mechanism also serves for performing fail-safe operation in response to occurrence of failure in the servo mechanism.
In such prior proposed throttle control system, a potentiometer has been used as the detector for detecting the stroke of the linkage and whereby detecting operation magnitude of the accelerator. The potentiometer is subject to secular variation resulting inaccuracy in monitoring the operation magnitude of the accelerator and, in turn, throttle valve angular position. In certain case, the secular variation may occur to increase internal resistance of the potentiometer, which resistance may interfere quick response to variation of the accelerator position to cause delay in production of the electric command ordering servo mechanism operation in a direction commanded through the accelerator. This clearly degrades the drivability of an automotive internal combustion engine. Furthermore, such delay will create certain difficulty in providing high response characteristics of fail-safe function.